In a plate type heat exchanger, the heat exchanging portion is constituted, for example, by laminating alternately plates and fins and brazing the portions where each plates contact with fins. Also, it is possible to form uneven portions for stirring on the flat surface of a plate and thus to omit the fins. In this case, the contacting portion of each plates is brazed.
As the brazing filler metal used for brazing, brazing copper alloy which is rich in practical application in various fields has been widely used. However, in heat exchangers requiring high temperature characteristics and excellent corrosion resistance, brazing nickel alloy superior in these features to brazing copper alloy in general has come to be also used.
As brazing nickel alloy is poor in processability, when brazing nickel alloy in powder form is applied for a plate type heat exchanger, the brazing alloy powder is adhered on the brazing parts such as the convex portions or flange portions formed on the plate or on the plate surface contacting with the fins and it is heated to braze.
Heretofore, as methods for adhering the brazing alloy powder on such a plate surface, the following methods have been known; that a paste prepared by mixing previously a brazing filler metal with a liquid binder for fixing the brazing filler metal is adhered by screen printing, that a paste brazing filler metal is adhered by using a dispenser, and that a brazing filler metal and a binder are sprayed simultaneously and adhered. Also, as the binder, an aqueous binder has been widely used in consideration of safety and environmental problems.
Recently, a heat exchanger excellent in corrosion resistance and strength characteristics has come to be required for further improvement on reliability and hence a brazing nickel alloy excellent in corrosion resistance and high in brazing strength has come to be required to satisfy the requirement.
However, the nickel-chromium-boron-silicon alloy (for example, JIS Z 3265 BNi-2) widely used as the brazing nickel alloy has a problem that it is poor in corrosion resistance though high in brazing strength. As it is also poor in fluidity, corrosion in the vicinity of the boundary between the brazing filler metal and the parts to be brazed had come to be a problem.
JP 2000-171188 A discloses a plate type heat exchanger brazed with a brazing nickel-chromium-phosphorus alloy excellent in fluidity is disclosed. Also, in JP 2000-190069 A, a method for the preparation of a plate type heat exchanger by using a brazing nickel-chromium-phosphorus alloy is disclosed. However, BNi-7 prescribed in JIS standard as a typical brazing nickel-chromium-phosphorus alloy had problems that it was lower in brazing strength and poorer in corrosion resistance than other brazing nickel alloys.
An object of the present invention is to provide a plate type heat exchanger high in pressure resistance and excellent in corrosion resistance and a method for its preparation.